Field of the Invention
This invention relates generally to method and apparatus for illumination and image projection by an active reflecting mini-optics system of a dynamic ensemble of mini-mirrors. The rotatable elements of this invention are mirrored balls and cylinders. Our system can even produce moving color images from a white light input containing no image information. This contrasts with other schemes which may be characterized as “direct observation light and dark displays.” Further original applications of this invention include interior and exterior lighting, a new kind of spotlight or lighthouse beacon, a building illumination system, a space-based light source for earth illumination, a reflected projection display system, and a low-cost large aperture telescope. Furthermore, the instant invention also teaches active elements such as ferrofluids which operate totally differently than the prior art.
The presence of rotatable mirrors in illumination and image projection presents either a dilemma or an opportunity with respect to the basic nature of the alignment implementation. Mirrors are normally made of a conductive metallic coating. In an applied electrostatic field, E, a dipole moment is induced in the metallic conducting material of the micro-mirrors because the charge distributes itself so as to produce a field free region inside the conductor. To internally cancel the applied field E, free electrons move to the end of each conducting mirror antiparallel to the direction of E, leaving positive charge at the end that is parallel to the direction of E. Another way to think of this in equilibrium is that a good conductor cannot long support a voltage difference across it without a current source. An induced electrostatic dipole in a pivoted conductor in an electrostatic field is somewhat analogous to an induced magnetic dipole in a pivoted ferromagnetic material in a magnetic field, which effect most people have experienced. When pivoted, a high aspect ratio (length to diameter ratio) ferromagnetic material rotates to align itself parallel to an external magnetic field.
If alignment is attempted in a conventional manner such as is used in Gyricon displays, the induced polarization electric dipole field in a mirror presents a dilemma since it is perpendicular to the zeta potential produced dipole field and the net vector is in neither direction. The “zeta potential,” is the net surface and volume charge that lies within the shear slipping surface resulting from the motion of a body through a liquid. The zeta potential is an electrical potential that exists across the interface of all solids and liquids. It is also known as the electrokinetic potential. The zeta potential produces an electric dipole field when a sphere is made from two dielectrically different hemispheres due to their interaction with the fluid surrounding it.
One way to eliminate or greatly diminish the effect of the zeta potential is to make the surface of both hemispheres out of the same material. This would be quite difficult for Gyricon displays because they require optically different surfaces e.g. black and white, or e.g. cyan, magenta, and yellow for color mixing. In the instant invention, no problem arises by making both hemispheres out of the same transparent material to eliminate or minimize the zeta potential. In fact this presents an opportunity to both utilize the induced polarization electric dipole field and to have two mirror surfaces. With two mirror surfaces, an option presents itself to use the better surface as the surface that reflects the light, and furthermore to have a standby mirror in each element should one of the mirrors degrade. A permanent electret dipole can be sandwiched between the two induced dipole mirrors to further enhance the dipole field that interacts with the addressable alignment fields.
The topic of the dipole interactions between balls seems not to have been discussed in the Gyricon patents and literature. A heuristic analysis shows that this is not a serious problem. The electric field strength of a dipole, Ed is proportional to 1/r3, where r is the radial distance from the center of the dipole. The energy in the field is proportional to (Ed)2. Thus the energy of a dipole field varies as 1/r6. The force is proportional to the gradient of the field, and hence varies as 1/r7. With such a rapid fall off of the dipole interaction force, it can generally be made very small compared to the force due to the applied field E, and to the frictional forces that are normally present. Therefore interaction of the dipole field forces between mirrored elements (balls or cylinders) can generally be made negligible.
Advantage of Focusing
A presently preferred maximum for the diameter of elements 1 is ˜10 mm or more. The minimum diameter of elements 1 can be assessed from the Rayleigh limit
      d    =                                        0.61            ⁢            λ                                n            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            u                          ~        10            ⁢      λ        ,where d is the minimum diameter of elements 1, λ˜4000 Å is the minimum visible wavelength, n is the index of refraction ˜1 of element 1 (the medium in which the incident light is reflected), and u is the half angle of the light beam admitted by elements 1. Thus d˜40,000 Å (4×10−6 m) is the minimum diameter of elements 1.
If the focussing planar mini-mirrors concentrate the incident light by a factor of 100, the total increase in power density at a receiving surface is 100 times greater than directly incident light from the same distance. Thus a much brighter image or illumination is possible than just from the light source alone.
Color Production
Unlike gyricon displays, the top half of the balls or cylinders is transparent or translucent so the incident light can reach the mirrors in the midplane. In gyricon displays the balls are made of opaque material which reflects light diffusely, rather than specularly as in the instant invention. Tinted transparent (translucent) top halves of the balls or cylinders in the instant invention differ substantially from the painted opaque gyricon ball surfaces. For color production in the instant invention, colored light may transmit through transparent top halves to reflect at the mirrors; or white light may enter through colored translucent top halves to reflect at the mirrors and leave the balls as primary colors for mixing. Primary colors are three colors such as red, green, and blue; or red, yellow, and blue; or cyan, magenta, and yellow; etc which can be combined (mixed) in various proportions to produce any other color. Contrary to common misunderstanding, the choice of primaries is somewhat arbitrary.